Apparatus and method for characterizing glass sheets

ABSTRACT

Disclosed is an apparatus and method for characterizing attributes of a moving glass sheet comprising complementary mechanical material handling technologies that progressively stabilize, position, capture, flatten, and release the lower portion of glass sheets traveling past the apparatus while posing minimal constraint on the top section of the sheet. The apparatus includes a pressure-vacuum (PV)-type device comprising distinct regions such that the glass sheets experience a non-contact but gradual increase in constraining force until the point where measurements can be performed, then a gradual decrease in constraining force until the glass sheets are released from the inspection station. This graduated force technique is applied along the direction of travel of the sheets and may also be applied vertically upwards along the height of the sheet to restrict the motion of the sheet without constraining it at pinch points near the conveyor.

This application is a divisional of U.S. patent application Ser. No.13/591,994 filed on Aug. 22, 2012, now U.S. Pat. No. 8,773,656 which inturn claims the benefit of priority under 35 U.S.C. §119 of U.S.Provisional Application Ser. No. 61/526,860 filed on Aug. 24, 2011 thecontents of which applications are relied upon and incorporated hereinby reference in their entirety.

BACKGROUND

1. Field

The present invention relates to an apparatus and method forcharacterizing glass sheets, and in particular to an apparatus adaptedto measure one or more selected attributes of a glass sheet while theglass sheet is in motion.

2. Technical Background

The invention relates to controlling motion and attitude of a glasssheet being conveyed along a predetermined path to enable highresolution online measurements, such as topography inspection (e.g.nanotopography, or topography on a nanometer scale). Accurate onlinemeasurements of thin glass sheets, and in particular the measurement ofnanometer-scale features, is highly dependent upon consistentlypresenting the glass sheets in a predetermined plane at a predeterminedorientation and eliminating the vast majority of vibrations andoscillations of the glass. Challenges such as high accuracy measurementof thickness, waviness, and cord are highly dependent on presenting theglass surface to measurement gauges with reproducible, high tolerancematerial handling.

Online process control and quality measurements, such as online cord andstreak inspection, suffer from a lack of repeatability when performedwhile simply gripping the glass in a non-quality area. A large part ofthe material presentation challenge is keeping the sheet of glass movingdown a production line while being suspended from a carrier on anoverhead conveyor. This constraint often forces measurements to beperformed either in a separate inspection portion of the process line,completely offline, or the measurement technology compatible with coarsetraditional online material handling is limited in its performance (e.g.resolution).

High resolution metrology is performed online in other industries, suchas silicon wafers or paper and plastic webs, but in these cases, theproduct is in direct contact with a supporting plate, as with wafers, orwith rollers, as with most webs. The size and contact prohibition onglass sheets suitable for display applications presents a difficultchallenge for handling.

SUMMARY

Measurement of thin glass sheets, generally equal to or less than 1 mmin thickness, may exhibit an amount of curvature or warping that createsdifficulty when measuring certain attributes of the glass, particularlyif the glass is large (e.g. greater than about 4 m²). To overcome thisdeficiency, the glass must first be flattened. In the past, flatteningand stabilizing the glass sheet has involved removing the sheet from theinline path, transferring the glass sheet to a precision granite base,then vacuuming individual glass sheets to a vacuum table, making thedesired measurements, removing the glass sheets, and then performing thesame operation with another glass sheet. Such a piecemeal approach addsconsiderable time and expense to a manufacturing process. The challengeof measuring large thin glass sheets is exacerbated if there is a needto measure the glass sheet while the glass sheet is transported along amanufacturing line.

In some manufacturing processes, glass sheets can be conveyed from onelocation to another location by clamping the glass sheet to a movingmember in an overhead conveyor. It would be beneficial if one or more ofthe aforementioned characterizations could be accomplished while theglass sheet was in motion, without first dismounting the glass sheet andpositioning the glass sheet on a measurement table as a stationaryobject.

To that end, an apparatus is disclosed for making precision measurementsof moving glass sheets, such as glass sheets suitable for use in aliquid crystal display devices, by constraining the glass sheets whilestill held by a conveyor carrier. The material handling features of theapparatus include air knives and pressure-vacuum (PV) air bearings,arranged in linear fashion such that a glass sheet entering theapparatus is subjected to a non-contact but gradual increase inconstraining force until the point where measurements can be performed.A gradual decrease in constraining force then occurs until the sheet isreleased from the apparatus. This graduated force technique is appliedalong the direction of travel of the glass sheets and also verticallyupward along the height of the sheets to restrict the motion of thesheets without constraining it at pinch points near the conveyorcarriers.

Accordingly, an air bearing is disclosed comprising an annular innerporous body portion comprising a circular groove in a surface of theinner porous body portion, and a plurality of radial groovesintersecting the circular groove, the inner porous body portion defininga central passage extending through a thickness of the air bearing; anouter porous body portion disposed about the inner porous body portion,wherein the outer porous body portion comprises a plurality ofcontinuous grooves in a surface of the outer porous body portion; andwherein each continuous groove of the outer porous body portioncomprises a plurality of vacuum ports. The circular groove of the innerporous body portion and the radial grooves of the inner porous bodyportion divide the surface of the inner porous body portion into aplurality of sub-surfaces, and a sub-surface of the plurality ofsub-surfaces comprises a vacuum port. Preferably, each sub-surface ofthe plurality of sub-surfaces comprises a vacuum port.

The outer porous body portion preferably comprises an arcuate outercircumference, and preferably the outer porous body portion comprises acircular outer circumference such that the air bearing comprises anannular inner porous body portion and an annular inner porous bodyportion disposed about and concentric with the inner porous bodyportion. In some embodiments the air bearing comprises a plurality ofinner porous body portions. For example, the plurality of inner porousbody portions may be aligned along a horizontal axis.

In another embodiment, an apparatus for characterizing glass sheets asthe glass sheets move past the apparatus is disclosed comprising: an airbearing comprising an annular inner porous body portion, and an outerporous body portion disposed about the inner porous body portion, theinner porous body portion defining a central passage extending through athickness of the air bearing; a plurality of stabilizing air knivespositioned upstream of the air bearing relative to a direction of travelof the glass sheets; and a measurement device to measure at least oneattribute of the glass sheet, the measurement device being aligned withthe central passage of the air bearing. The inner porous body portioncomprises a circular groove in a surface thereof. The inner porous bodyportion may further comprise a plurality of radial grooves intersectingthe circular groove. The surface of the inner porous body portioncomprises a vacuum port. If the inner porous body portion comprises acircular groove and a plurality of radial grooves, the circular grooveand the radial grooves define a plurality of sub-surfaces on the innerporous body portion. Preferably, each sub-surface comprises a vacuumport.

The outer porous body portion comprises a plurality of continuous (i.e.closed) grooves, each continuous groove comprising a plurality of vacuumports. For example, each continuous groove can be a circular, oval,elliptical, or any other closed, continuous shape. Preferably, an outercircumference of the outer porous body portion is arcuate. For example,the outer circumference of the outer porous body portion may becircular. The air bearing may in some embodiments comprise a pluralityof inner porous body portions.

The measurement device preferably measures the at least one attributethrough the passage defined by the inner porous body portion.

The stabilizing air knives are oriented such that a flow of air from thestabilizing air knives is angled in a downward direction. That is, theflow of air from the stabilizing air knives is preferably directed in adownward direction relative to a horizontal plane so that the flow ofair makes an acute angle with the glass sheet. For example, a directionof the flow of air may form an angle in a range from about 15 degrees toabout 75 degrees relative to the glass sheet. Preferably, thestabilizing air knives are arcuate in shape.

The apparatus according to the present embodiment may further comprise apositioning air knife positioned downstream of the air bearing to forcethe glass sheet in a direction away from the air bearing.

In still another embodiment, a method of characterizing moving glasssheets is described comprising: moving a glass sheet in a firstdirection along a predetermined path, the glass sheet comprising a pairof opposing major surfaces, a bottom edge, and a leading edge relativeto the first direction; dampening movement of the glass sheet in asecond direction perpendicular to the first direction by passing theglass sheet between at least two stabilizing air knives as the glasssheet is moving in the first direction; engaging the glass sheet with acircular air bearing, the circular air bearing comprising an innerporous body portion and a outer porous body portion disposed about theinner porous body portion, the inner porous body portion defining acentral passage therethrough; and measuring at least one attribute ofthe glass sheet as the glass sheet moves in the first direction. Themethod may further comprise guiding a bottom edge of the glass sheetwith an edge guiding device comprising guide arms arranged to form a“V”-shaped slot therebetween.

A height of the air bearing, and more particularly a height of the outerporous body portion, is less than one half a height of the glass sheet.The air bearing is positioned such that an upper one half of the glasssheet is preferably not adjacent to the air bearing as the glass sheetis measured. The air bearing is preferably capable of maintaining theglass sheet within +/−15 μm of a predetermined distance from the innerporous body portion.

The first inner porous body portion comprises a circular groove in asurface thereof. The inner porous body portion preferably also comprisesa plurality of radial grooves in the surface of the inner porous bodyportion, wherein the plurality of radial grooves intersect the circulargroove.

The outer porous body portion of the air bearing preferably comprises aplurality of concentric grooves, wherein each groove of the plurality ofconcentric grooves comprising a plurality of vacuum ports.

In still another embodiment, a method of making a glass sheet isdescribed comprising: heating a batch material in a melting furnace toform a molten glass material; flowing the molten glass material overconverging forming surfaces of a forming body to produce a glass ribbon;cutting a glass sheet from the glass ribbon; suspending the glass sheetvertically from a conveyor, the conveyor moving the glass sheet in afirst direction along a predetermined path; dampening movement of theglass sheet in a second direction perpendicular to the first directionby passing the glass sheet between at least two stabilizing air knivesas the glass sheet is moving in the first direction; engaging the glasssheet with an air bearing, the air bearing comprising an annular innerporous body portion and a outer porous body portion disposed about theannular inner porous body portion, the annular inner porous body portiondefining a central passage therethrough; and measuring at least oneattribute of the glass sheet through the central passage as the glasssheet moves in the first direction. The outer porous body portionpreferably comprises an arcuate outer circumference, such as a circularouter circumference.

The inner porous body portion comprises a circular groove in a surfacethereof, and further preferably comprises a plurality of radial groovesintersecting the circular groove. The outer porous body portioncomprises a plurality of continuous grooves in a surface thereof.

The method may further comprise using a first positioning air knife tomove the glass sheet in a direction away from a leading edge of the airbearing. In a further optional step, the method may further compriseusing a second positioning air knife to move the glass sheet in adirection toward the air bearing. In still another optional step, themethod may further comprise using a third positioning air knife to movethe glass sheet away from a trailing edge of the air bearing.

Each stabilizing air knife of the at least two stabilizing air knivesdirects a flow of air in a downward direction. A leading end of astabilizing air knife of the at least two stabilizing air knives mayoptionally be pitched or inclined downward relative to a trailing end ofthe stabilizing air knife.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and constitute a part of this specification. The drawingsillustrate various embodiments of the invention and, together with thedescription, serve to explain the principles and operations of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary fusion glass making systemfor producing glass sheets;

FIG. 2 is a perspective view of an apparatus for characterizing a glasssheet according to an embodiment of the present invention;

FIG. 3 is a view of the apparatus of FIG. 2 seen looking down on theapparatus;

FIG. 4 is a top down view of an edge guiding device according to anembodiment of the present invention;

FIG. 5 is a top down view of an edge constraining device according to anembodiment of the present invention;

FIG. 6 is a side view of a roller of the edge constraining device ofFIG. 5 showing the roller engaged with a rotary encoder;

FIG. 7 is a simplified front view of an air bearing according to anembodiment of the present invention illustrating the inner porous bodyportion and the outer porous body portion;

FIG. 8 is a side (edge) view of the air bearing of FIG. 7;

FIG. 9 is a front view of an air bearing according to an embodiment ofthe present invention shown in relation to a position of the air bearingrelative to a glass sheet;

FIG. 10 is a detailed view of the air bearing of FIG. 7;

FIG. 11 is a frontal view of another embodiment of an air bearingwherein the air bearing comprises a plurality of inner piorous bodyportions;

FIG. 12A is a cross sectional view of a portion of the inner porous bodyportion of the air bearing of FIG. 10;

FIG. 12B is a cross sectional view of a portion of the outer porous bodyportion of the air bearing of FIG. 10;

FIG. 13 is a perspective view of an exemplary linear stabilizing airbearing according to the present invention, illustrating the flow of airfrom the elongated nozzle in a planar fashion;

FIG. 14 is a front view of the air bearing of FIG. 7 illustratingdownward angle of an exemplary stabilizing air knife;

FIG. 15 is a top down view of the air bearing of FIG. 7 illustrating asideways angle of an exemplary stabilizing air knife;

FIG. 16 is a side (edge) view of the air bearing of FIG. 7 illustratinga downward of a flow of air issuing from an exemplary stabilizing airknife;

FIG. 17 is a front view of the air bearing of FIG. 7 depicting theleading and trailing edges of the air bearing in relation to thedirection of travel of the glass sheet;

FIG. 18 is a top down view of the apparatus of FIG. 2 showing thecurvature produced in a glass sheet by the apparatus;

FIG. 19 is a cross sectional side view of a portion of the air bearingof FIG. 17 showing details of the curvature of the glass sheet adjacentto the air bearing;

FIG. 20 is a graph of fly height vs. time for two locations on the glasssheet as the glass sheet traveled at a speed of 100 mm/s, andillustrating the stability of the glass sheet position (i.e. fly heightconsistency).

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

In a down draw glass sheet making process, such as a fusion down drawprocess for example, glass sheets are formed by drawing a viscous glassmaterial vertically downward under suitable conditions of viscosity anddraw rate to form a ribbon of glass. The ribbon of glass comprises aviscous liquid at the upper-most extreme position of the ribbon andtransitions from a viscous liquid to a solid glass ribbon as thematerial passes through the glass transition temperature range. When thedescending bottom portion of the ribbon has reached a suitabletemperature and viscosity, a glass sheet is cut from the ribbon, and sothe process continues, with the glass sheets being cut from acontinuously descending glass ribbon.

An exemplary fusion down draw glass making system 6 is shown in FIG. 1.In accordance with FIG. 1, batch materials 8 are loaded into meltingfurnace 10 and heated to form viscous molten glass material 12. Moltenglass material 12 is conveyed through finer 14 where bubbles are removedfrom the molten glass material, and then stirred in stirring apparatus16 to homogenize the molten glass material. The stirring operation seeksto eliminate variations in the chemical consistency of the molten glassmaterial, thereby avoiding variations in the physical and opticalproperties of the final glass. Once the molten glass material has beenstirred, it flows through receiving vessel 18 and then to forming body20. Receiving vessel 18 functions as an accumulator by dampening minorflow fluctuations. Forming body 20 comprises a ceramic body having anopen channel 22 in an upper portion of the body, and a pair ofconverging exterior forming surfaces 24 that join at a bottom of theforming body. The molten glass material overflows the open channel ofthe forming body and flows down the converging forming surfaces of theforming body as two separate flows of molten glass material. Theseparate flows of molten glass material join and form a single flow orribbon 26 of material where the converging forming surfaces cometogether. The ribbon cools as it descends through the glass transitiontemperature region and forms a solid glass ribbon from which glasssheets 28 are cut along cut line 29.

Melting furnace 10 is connected to and in fluid communication with finer14 through melter-to-finer connecting tube 30, and finer 14 is connectedto and in fluid communication with stirring apparatus 16 throughfiner-to-stirring apparatus connecting tube 32. Stirring apparatus 16 isconnected to and in fluid communication with receiving vessel 18 throughstirrer-to-receiving vessel tube 34, and receiving vessel 18 isconnected to and in fluid communication with forming body 20 throughdowncomer tube 36 and forming body inlet 38. While melting furnace 10 istypically formed from a ceramic material, such as ceramic bricks (e.g.alumina or zirconia), those components involved in transporting andprocessing the molten glass material are typically formed from platinum,or a platinum alloy such as a platinum-rhodium alloy. Thus,melter-to-finer connecting tube 30, finer 14, finer-to-stirringapparatus connecting tube 32, stirring apparatus 16, stirringapparatus-to-receiving vessel tube 34, receiving vessel 18, downcomertube 36 and forming body inlet 38 typically comprise platinum or aplatinum rhodium alloy.

Since the glass sheets begin as vertically oriented sheets when they areremoved from glass ribbon 26, reduced handling is possible if the glasssheets can be maintained in a vertical orientation as they aretransported through at least a portion of the manufacturing processdownstream of the forming process. Thus, in certain manufacturingprocesses the glass sheet is attached to and supported from a raisedconveyor after being cut from the ribbon and moved through at least aportion of the process line in a vertical orientation. In addition, itis more efficient to perform post-forming processing while the glasssheet is traveling rather than dismounting the sheet, placing the sheetin a fixture, processing the sheet, remounting the sheet andtransporting it to a subsequent process. To that end, an apparatus isdisclosed herein for measuring characteristics of a glass sheet afterthe sheet has been cut from the ribbon and as the glass sheet is moving.Measured characteristics can include cord, streak or thickness. Cordrelates to a compositional inhomogeneity in the bulk glass. Thiscomposition inhomogeneity can lead to periodic nanometer-scaletopography deviations. In the liquid crystal display (LCD) field, thesedeviations can lead to periodic cell gap variations in the display panelitself, which in turn lead to contrast streaks to which human perceptionis finely attuned. Streaks can cause the same distortions in LCD panelsbut are caused by flow distortions on the body used to form the glasssheet. In accordance with FIG. 2, before entering apparatus 40, theglass sheet is transported by securing the glass sheet only at the upperedge of the glass sheet so that the glass sheet hangs freely from thissupport.

While the preceding brief description is focused on a fusion down drawglass sheet manufacturing process, the present invention is not limitedto a fusion down draw process, and could be practiced in other glasssheet manufacturing processes, such as a slot draw process.

FIGS. 2 and 3 depict an exemplary embodiment of an apparatus 40 formeasuring characteristics of a glass sheet. As shown in both FIGS. 2 and3, apparatus 40 comprises a frame 42 supporting a circular air bearing44 in an upright, vertical orientation. Air bearing 44 is apressure-vacuum device designed to maintain a substrate, such as a glasssheet, at a predetermined distance, and within a maximum deviation, fromthe surface of the air bearing. The predetermined distance is referredto as the fly height. The fly height represents an equilibrium positionof the substrate relative to the air bearing. As air is drawn frombetween the glass sheet and the air bearing through one or more vacuumports, ambient air pressure forces the substrate toward the air bearing.However, as the substrate moves toward the air bearing, the forceagainst the substrate produced by the air issuing from the poroussurface(s) of the air bearing increases, until the substrate reaches aposition where the forces are in equilibrium. Thus, the substrate iscaptured and held by the air bearing. The fly height exhibits somedeviation about a given nominal fly height. As used herein, a vacuumport is any opening within a surface of air bearing 44 in fluidcommunication with a passage, such as a pipe, tube or other structurefor the conveyance of a gas, and connected with or intended forconnection to a vacuum source, such as a vacuum pump. Vacuum ports maybe interconnected, such as through a common plenum disposed within airbearing 44, through a common plenum external to air bearing 44, or beindividually supplied with a vacuum.

Air bearing 44 comprises a major surface 46, which is the surfaceclosest to the adjacent glass sheet to be measured, comprising channelsor grooves, and vacuum ports, as will be discussed in more detail below.For the purpose of clarity, reference to angular locations on majorsurface 46 will be made in reference to an outer circumference of thecircular air bearing, with a position of 0 degrees being located at thetop-most point of the air bearing, and increasing angular positionrelative to a center of the circular air bearing occurs in a clockwiserotation through 360 degrees.

Referring to FIG. 2, conveyor 48 may be used to transport a glass sheetin a direction of travel 50 along a predetermined path through apparatus40 so that a measurement of the glass sheet may be made. For example,conveyor 48 may comprise an overhead rail equipped with a clampingmechanism 49 that travels along the rail and which also clamps to a topedge of the glass sheet to be measured. Preferably the clampingmechanism is configured to roll or slide along the rail. Moreover,conveyor 48 is preferably equipped with a drive mechanism that moves theclamping mechanism, and the glass sheet, along the rail assembly. Forexample, the rail assembly may be fitted with a driven chain or beltconnected to the clamping mechanism, wherein a motor or other motiveforce is used to move the chain or belt, thereby causing the clampingmechanism, and therefore the glass sheet clamped by clamping mechanism49, to traverse along the rail assembly and through apparatus 40. Asused herein, the direction of travel 50 represents a forward movement ofthe glass sheet through apparatus 40. In addition, the terms upstreamand downstream are used relative to direction of travel 50. That is,upstream is to be construed as a direction generally opposite todirection of travel 50, whereas downstream is to be construed as beingin a direction generally the same as direction 50. However, it should benoted that upstream and downstream designations do not require that thedirection referred to is identical, or exactly opposite the direction oftravel 50. It is only required that the direction referred to has novector component in the opposite direction. For example, an upstreamdirection has no downstream vector component. Additionally, upstream anddownstream may be used to refer to a stationary position with respect tothe moving glass sheet. In this respect, upstream refers to a positionthat encounters the moving glass sheet first in respect of anotherstationary position. Thus, the glass sheet traveling in direction oftravel 50 may pass one fixed point or object before passing a subsequentpoint or object. The first-passed point or object is referred to as theupstream point or object relative to the subsequent point, whereas thesubsequent point or object is the downstream point or object relative tothe first-passed point.

Apparatus 40 further comprises a plurality of glass sheet stabilizingair knives comprising a first stabilizing air knife 52 a positioned suchthat the first stabilizing air knife will be located opposite a firstmajor surface of the glass sheet (where the first major surface of theglass sheet is the glass surface closest or adjacent to the airbearing), and a second stabilizing air knife 52 b located opposite asecond major surface of the glass sheet. Put more simply, one air knifeis positioned adjacent to one side of the glass sheet while the otherair knife is positioned adjacent to the other or opposite side of theglass sheet. Additional glass sheet stabilizing air knives 52 a, 52 bmay be positioned such that they are opposite the first or second majorsurface of the glass sheet as needed. For example, FIGS. 2 and 3 depictfour pair of stabilizing air knives arrayed in rows and column.Apparatus 40 may comprise additional, positioning air knives positionedupstream of air bearing 44, downstream of the air bearing and/oropposite the air bearing to assist in positioning the glass sheetrelative to the air bearing, as described in more detail further below.

Apparatus 40 also comprises an edge guiding device 54 for guiding glasssheets into position for measurement. Edge guiding device 54 is locatedupstream of the stabilizing air knives relative to the travel directionof the glass sheet and functions to reduce or eliminate side-to-sidesway of the glass sheet and to guide the glass sheet between thestabilizing air knives. In the embodiment shown in FIGS. 2 and 3 and asperhaps best shown in FIG. 4, edge guiding device 54 comprises a pair ofguide arms 56 a, 56 b configured to form guide slot 58 for receiving thelower edge of the glass sheet. Preferably guide slot 58 is wedge orV-shaped, where a distance d between the guide arms at an inlet(upstream) end of guide slot 58 (relative to glass sheet direction oftravel 50) where a glass sheet enters the guide slot is greater than adistance d′ between guide arms 56 a, 56 b at an outlet (downstream) endof the guide slot. More simply put, the distance between the guide armsvaries along a length of the guide slot and direction 50 such that theguide slot narrows as a glass sheet progresses through the guide slot,thereby forming a V-shaped slot that narrows in a direction toward airbearing 44. Preferably, as shown in the embodiment of FIG. 4, each guidearm may be rotatably mounted to frame 42 by axle pins or bolts that areinserted into a complimentary hole 60 in each guide arm and fasten toframe 42. Alternatively, each guide arm may comprise a pin that is fitwithin a complimentary hole in frame 42. Thus, the guide arms can berotated to vary a shape of guide slot 58. Means for locking the guidearms is also preferably provided, thereby allowing each guide arm to beimmobilized when a suitable slot shape is implemented. For example, theguide arms may be fitted with clamps or locking screws. In someembodiments, apparatus 40 may comprise a plurality of edge guidingdevices 54. The width d of guide slot 58 should be sufficient toaccommodate the largest anticipated side-to-side movement, or sway, ofthe glass sheet (where the glass sheet is rotating about clampingmechanism 49) to facilitate capture of the glass sheet. For example, ifd is not sufficiently wide, a swaying glass sheet may not be capturedwithin guide slot 58 but instead conveyed into contact with elements ofapparatus 40, thereby potentially damaging the glass sheet or theapparatus. Width d will be dependent on the parameters of a particularprocess configuration. As described, width d′ is smaller than width d,should be sufficiently large to prevent binding of the glass sheet as ittravels through guide slot 58, but should also be sufficiently narrowthat side-to-side sway is reduced or eliminated. Width d′ is dependent,for example, on the thickness of the glass sheet and the magnitude ofany curvature exhibited by the sheet. Alternatively, edge guiding device54 may be a block of material comprising a slot machined into an uppersurface of the block.

Apparatus 40 preferably also comprises an edge constraining device 62,best seen in FIG. 5, to hold the lower edge 63 of the glass sheet as theglass sheet is moving in the first direction 50 adjacent the airbearing. For example, edge constraining device 62 can be a plurality ofguide rollers comprising one or more pairs of opposing rollerspositioned along the path of the glass sheet as it traverses apparatus40. In accordance with the embodiment of FIG. 5, each roller paircomprises a fixed position roller 64 and an opposing movable roller 66.The fixed roller of a roller pair is configured to be rotatable, but nototherwise movable. That is, while fixed position roller 64 can rotateabout an axis of rotation of the roller, it is not adapted to translateor swing (describe an arc). On the other hand, the opposing movableroller 66 of the roller pair is configured to be both rotatable and tobe movable (e.g. translatable) such that a distance between a fixedposition roller 64 and an opposing movable roller 66 can vary.Preferably, movable roller 66 is urged toward fixed position roller 64,such as with a spring 68. As shown by FIG. 5, movable roller 66 iscoupled to a pivot arm 70 that pivots about a pivot point 72. Spring 68is compressed between pivot arm 70 and spring stop 74, whereby movableroller 66 is urged toward fixed position roller 64. A glass sheet 28that is inserted between fixed position roller 64 and movable roller 66causes movable roller 66 to rotate about pivot point 72 and describe acircular arc centered about pivot point 72. A simultaneous movement ofpivot arm 70 against spring 68 further compresses spring 68. That is,the axis of rotation of movable roller 66 itself rotates about pivotpoint 72. Accordingly, movement of movable roller 66 away from fixedposition roller 64 is resisted by the force exerted by spring 68 throughpivot arm 70, and movable roller 66 is urged against glass sheet 28 sothat glass sheet 28 is pinched between the fixed roller and the movableroller.

To track progress of the glass sheet through apparatus 40, at least oneroller of constraining device 62 may include a rotary encoder device tosense the rotational movement of the rotating roller and convert therotational movement to an electrical signal. FIG. 6 depicts a side viewof a fixed position roller 64 comprising roller axle 76 and rotaryencoder 78 coupled to the roller through roller axle 76 and drive belt80. Other methods of coupling rotary encoder 78 may be employed, as areknown in the art. Rotary encoder 78 rotates as a ratio of the rotationof the roller and develops or modifies an electrical signal 79. Thedeveloped or modified electrical signal 79 from rotary encoder 78 maythen be conveyed to a receiving computational device (not shown), wherea linear movement of the glass sheet can be calculated using therotational data from the rotary encoder. As best shown in FIG. 6, eachfixed position roller 64 and each movable roller 66 comprises aresilient surface 82 to prevent damage resulting from contact betweenthe roller and the glass sheet.

Referring now to FIGS. 7 and 8, air bearing 44 comprises a porous body84 comprising generally planar major surface 46. As used herein porousmeans a rigid but sponge—like material in that it comprises millions ofminute random channels through the thickness of the material resultingin a uniform distribution of holes at an external surface thereof, eachhole almost insignificant by itself. However, when the porous materialis supplied with a gas under pressure the holes together supply asubstantially uniform flow of air from a surface of the material. Asuitable porous material in keeping with the present definition isgraphite. Other materials, such as sintered metal powders may also beused, but the added risk of scratching the glass surface due to the hardabrasive nature of the metal argues for a softer material, such asgraphite. As shown in FIG. 9, the overall height D of porous body 84 istypically no more than one half the height H of the glass sheet 28 to bemeasured (where dashed line 88 represents H/2), and preferably theheight of porous body 84 is no greater than one third the height of theglass sheet, or less, where the height of the glass sheet is thedimension of the glass sheet in a vertical direction when the glasssheet is hanging vertically from conveyor 48. Moreover, as also shown inFIG. 9, it is preferable that during operation the air bearing ispositioned adjacent only the bottom portion of the glass sheet. That is,the air bearing is preferably positioned so that porous body 84 isadjacent only the lower one half or less of the glass sheet, or aportion thereof. If porous body 84 is positioned high on the glass sheet(e.g. above dashed line 88), the glass sheet may be subject to unduestress resulting from the constraint placed on the glass sheet by boththe conveyor clamping mechanism and the constraint applied by the airbearing. The resulting stress may break the glass sheet.

Returning to FIG. 7, porous body 84 is further divided into a first, orinner porous body portion 90 and a second, or outer porous body portion92 disposed about the inner porous body portion. Accordingly, planarmajor surface 46 is divided into an inner planar surface 94 comprisinginner porous body portion 90, and an outer planar surface 96 comprisingouter porous body portion 92. Inner planar surface 94 and outer planarsurface 96 may be coplanar.

Inner porous body portion 90 of air bearing 44 is annular in shape,having a circular inner circumference 98 defined at a radius r₁ from thecenter of the inner circumference and an outer circumference 100 definedat a radius r₂ from the center of the inner circumference. In addition,inner circumference 98 denotes the outer circumference of a passage 102extending through air bearing 44. In a typical embodiment, passage 102is in a range from about 3 cm to about 8 cm in diameter. However,passage 102 may be larger or smaller, depending on need and the natureof the measurement to be taken. Measurement device 104 (see FIG. 2) islocated such that air bearing 44 is positioned between glass sheet 28and measurement device 104 and so that an optical axis 105 of themeasurement device extends through passage 102. Such a “through”measurement is beneficial that the plane of inspection (fixed bymeasurement device 104) and the plane of the glass are coplanar. Opticalaxis 105, may, for example, coincide with the center of innercircumference 98 as shown in FIG. 7. In other embodiments, measurementdevice 104 may be positioned so that glass sheet 28 is betweenmeasurement device 104 and air bearing 44. Nevertheless, measurementdevice 104 should still be positioned such that optical axis 105 ofmeasurement device is aligned to pass through passage 102. However, incertain embodiments, passage 102 may be eliminated when the measurementis to be taken from the side of glass sheet 28 facing porous body 84 ifreflection of light from porous body 84 does not affect the quality of,or otherwise interfere with, the particular measurement being performed.Optical axis 105 of measurement device 104 may be, for example, a laserbeam emitted by the measurement device toward glass sheet 28.

Referring now to FIG. 10, inner porous body portion 90 comprises atleast one circular groove 106 concentric with inner circumference 98.Inner porous body portion 90 further comprises a plurality of grooves108 extending radially on inner planar surface 94 and intersecting withcircular groove 106. Radial grooves 108 are preferably arranged atperiodic angular positions in a spoke-like fashion. Circular groove 106and intersecting radial grooves 108 divide inner planar surface 94 intoa plurality of sub-surfaces 87. Each sub-surface 87 comprises at leastone vacuum port 110 in fluid communication with a vacuum source (notshown), as previously described.

Like inner porous body portion 90, outer porous body portion 92 of airbearing 44 is arcuate in shape, but need not possess a circular outercircumference. For example, outer porous body portion may be ellipticalor oval in shape. Outer porous body portion 92 is disposed about innerporous body portion 90 and comprises a circular inner circumference 112defined at a radius r₃ from the center of inner porous body portion 90described above. In embodiments wherein outer porous body portion 92comprises a circular outer circumference, i.e. circumference 114 shownin FIG. 7, outer circumference 114 is defined at a radius r₄ from thecenter of inner circumference 98. In some embodiments, r₂=r₃ andtherefore inner circumference 112 of outer porous body portion 92 is thesame as the outer circumference 100 of inner porous body portion 90.

Still in regard to FIG. 10, outer porous body portion 92 furthercomprises a plurality of continuous grooves 116 formed in outer planarsurface 96. Each continuous groove 116 comprises a plurality of vacuumports 118 extending through the porous body and connected to a vacuumsource. Preferably, the plurality of vacuum ports 118 are arrayedperiodically within each continuous groove 116 so that the angulardisplacement between vacuum ports disposed in a given continuous grooveis equal. For example, within a given continuous groove 116, a vacuumport 118 may be positioned every 5 degrees, every 10 degrees or every 15degrees around the groove. It is not necessary that the vacuum ports ofone continuous groove 116 coincide angularly with the vacuum ports ofanother continuous groove 116. In some cases, particularly when theouter circumference of outer porous body portion 92 is circular,continuous grooves 116 are preferably circular and concentric.

In some embodiments, such as that depicted in FIG. 11, air bearing 44may comprise a plurality of inner porous body portions 90 positionedwithin outer porous body portion 92, each inner porous body portiondefining a passage 102. This can be particularly helpful when multiplemeasurements, for simultaneously determining multiple characteristics ofthe glass sheet, are to be taken and cannot be incorporated into asingle measurement device.

The organization of grooves and vacuum ports can be better seen with theaid of FIGS. 12A and 12B, where FIG. 12A depicts a cross sectional viewof a portion of inner porous body portion 90, and FIG. 12B depicts across sectional view of a portion of outer porous body portion 92. Bothinner porous body portion 90 and outer porous body portion 92 aresupplied with a pressurized gas, such as air, that issues from theplanar surface of each porous body portion as represented by arrows 117.Together, the vacuum produced at the vacuum ports, depicted by arrows119, and the air pressure produced over the planar surfaces of theporous body portions define two zones: a low precision capture zoneadjacent outer planar surface 96 and a high precision capture zonecoincident with inner planar surface 94. In the low precision capturezone the fly height of the glass sheet may be greater than the flyheight of the glass sheet adjacent the high precision capture zone. Afly height of the glass sheet adjacent the low pressure capture zone cantypically by in the range from about 40 μm to 60 μm, whereas the flyheight of the glass sheet adjacent the high precision zone can typicallybe less than 40 μm.

As previously described, and in accordance with the embodiment of FIGS.2 and 3, apparatus 40 comprises a plurality of stabilizing air knives 52a, 52 b positioned upstream of air bearing 44 relative to the directionof travel 50 of glass sheet 28 through apparatus 40. The plurality ofstabilizing air knives comprises a first stabilizing air knife 52 apositioned such that the first stabilizing air knife will be locatedopposite first major surface 121 of glass sheet 28 (See FIG. 18), and asecond stabilizing air knife 52 b positioned such that the secondstabilizing air knife will be located opposite second major surface 123of glass sheet 28. First major surface 121 of glass sheet 28 is thesurface of the glass sheet closest to porous body 84 when the glasssheet is adjacent to the air bearing, whereas second major surface 123is the surface of glass sheet 28 farthest from porous body 84 under thesame condition. The flow of air from the at least first and secondstabilizing air knives of the plurality of stabilizing air knivesstabilizes lateral (side-to-side) motion of the glass sheet inconjunction with the at least one edge guiding device 54 as the glasssheet enters the space between the stabilizing air knives, and helps toflatten the sheet. More simply put, even though the glass sheet may beimpeded from lateral movement at the upper and lower edges of the glasssheet by conveyor clamping mechanism 49 at the upper edge of the glasssheet and edge guiding device 54 at the lower edge of the glass sheet,the glass sheet may still deform in a direction perpendicular to thegeneral plane of the glass sheet, much the way a cloth sail can billowin the wind. This is because the glass sheet can be very large, and verythin, giving the glass sheet an increased flexibility when compared tomuch thicker glass plates. For example, a thickness of the glass sheetcan be less than 1 mm.

Each stabilizing air knife is oriented such that the flow of air fromeach stabilizing air knife is directed toward the glass sheet in adownward direction, generally toward the bottom of the glass sheet, tocreate a more laminar flow of air over the major surfaces of the glasssheet and prevent turbulence and subsequent buffeting of the glasssheet. Preferably, although not necessarily, first and secondstabilizing air knives 52 a, 52 b are arranged to mirror each otheracross the glass sheet. For example, in some embodiments, such as theembodiment of FIGS. 2 and 3, the plurality of stabilizing air knives arepreferably arranged as multiple pairs of partially or substantiallyopposing air knives. That is, while the air knives may be directlyopposing each other, this is not necessary, and there may be some offsetbetween such “pairs” of air knives. However, in some embodiments theoffset may be substantial. The number of stabilizing air knives isprocess dependent, and will depend, for example, on the transport speedof the glass sheet, the size and weight of the glass sheet and theamount of side-to-side sway exhibited by a glass sheet in a particularmanufacturing process line. Similarly, the exact placement of an airknife on one side of the glass sheet compared to the placement ofanother air knife on the other side of the glass sheet will depend onthe particular process conditions of the installation.

FIG. 13 illustrates an exemplary stabilizing air knife (here designatedgenerally by reference numeral 120) comprising a generally elongate body122 having an elongate orifice 124 from which a flow 126 of air issues.For simplicity, the air knife is represented by a longitudinallyextended rectangular block. Each elongate orifice 124 is in fluidcommunication with a source of pressurized gas that enters the air knifethrough a coupling. The air knife may include an interior plenum influid communication with orifice 124. As air is quite satisfactory as agas, being both plentiful and essentially free, the remainingdescription will assume an air-based air knife. Each elongate body 122is arranged such that a direction of flow of the air emitted from eachelongate orifice 124 is at a downward angle relative to a referencehorizontal plane. Each stabilizing air knife, as represented byexemplary stabilizing air knife 120, includes a forward or leading end Land a rearward or trailing end TR relative to the direction of travel 50of the glass sheet. That is, the leading end of the air knife is fartherupstream than the trailing end of the air knife. When the air knife issupplied with pressurized air, the air issues from elongate orifice 124at a high velocity. While the air issuing from the elongate orifice 124may eventually begin to diverge after leaving the stabilizing air knife,for at least a short distance, on the order of several 10s ofmillimeters, the air issues from the air knife as a substantiallylaminar flow 126 that can be approximated by a plane. Exemplarystabilizing air knife 120 further comprises a top surface T.

In the event that the stabilizing air knives are arranged in acomplimentary opposing relationship (i.e. are mirrored across anintervening vertical plane between the stabilizing air knives that isparallel with air bearing major surface 46), a distance between theleading ends of an opposing stabilizing air knife pair may be greaterthan a distance between the trailing ends of the opposing stabilizingair knife pair. That is, the distance between the opposing air knivesnarrows as the glass sheet progresses between the air knives in a mannersimilar to the narrowing of guide slot 58.

In still another optional characteristic, each stabilizing air knife ofthe plurality of stabilizing air knives may be oriented such that thetrailing end of each stabilizing air knife is higher (or lower) than theleading end of the stabilizing air knife. In some embodiments eachstabilizing air knife can be straight (i.e. rectangular shaped) similarto exemplary air knife 120. However, preferably each stabilizing airknife is arcuate and may comprises a circular arc. Suitable stabilizingair knives of either the straight (linear) variety, or the arcuatedesign, can be obtained, for example, through Exair Corporation locatedin Cincinnati, Ohio, USA.

How each stabilizing air knife may be spatially oriented can bevisualized in more detail with the following description and aid ofFIGS. 14-16. The orientation of a body in three-dimensional spacerequires a frame of reference, and a means of orienting the body in thatframe of reference. FIG. 14 shows a vertical X-Y plane coplanar withmajor surface 46 of porous body 84. For the purpose of furtherdiscussion, this X-Y plane forms one plane in a three-dimensionalCartesian coordinate reference frame. This X-Y plane lies within theplane of the page on which FIG. 14 is depicted. A second vertical plane,seen from an edge thereof in FIG. 14, forms a Y-Z plane of the Cartesiancoordinate system where the Z direction extends perpendicular to andtherefore out of the page. The Y-Z plane is perpendicular to the X-Yplane. A third, X-Z plane, also seen from an edge thereof in FIG. 14, isarranged to be perpendicular to both the X-Y and Y-Z planes. For thepurpose of further discussion, and unless otherwise described, theorigin of the Cartesian coordinate system formed by the three planesX-Y, Y-Z and X-Z described above lies at the center of inner porous bodyportion 90, and this Cartesian coordinate system will be used todescribe the orientation of the air knives in three dimensional space.

FIGS. 14-16 depict the three optional orientations of exemplarystabilizing air knife 120, and by extension therefore the optionalspatial orientations of each stabilizing air knife, shown separately toaid in visualizing the orientations. FIG. 14 depicts an outline of airbearing 44 as seen looking at major surface 46 and indicating thedirection of travel 50 of the glass sheet. Exemplary stabilizing airknife 120 exhibits a downward pitch or incline in that the leading end Lof the stabilizing air knife is lower than the trailing end TR of thestabilizing air knife relative to the horizontal X-Z plane. To wit,plane 128 representing the flow of air from the exemplary stabilizingair knife makes an angle α with the X-Z plane.

FIG. 15 shows a second view looking down on an edge of air bearing 44and shows an edge of the Y-Z plane and an edge of the X-Y plane. The X-Zplane is perpendicular to both the X-Y plane and the Y-Z plane. Plane130 is a plane longitudinally bisecting top surface T of exemplarystabilizing air knife 120 and is perpendicular to plane 126 representingthe flow of air from the air knife. In accordance with FIG. 15,exemplary stabilizing air knife 120 may be angled relative to thevertical X-Y plane such that a non-zero angle β is formed between plane130 and the X-Y plane.

FIG. 16 shows a third view looking down on an edge of air bearing 44 andshows an edge of the X-Z plane and an edge of the X-Y plane. The Y-Zplane is perpendicular to both the X-Y plane and the X-Z plane. FIG. 16illustrates exemplary stabilizing air knife 120 from an end thereoforiented such that the flow of air exiting the air knife is directeddownward (from a reference horizontal flow, e.g. parallel with thehorizontal X-Z plane) and the plane of the air flow makes an acute angleδ with the X-Y plane rather than being directed, for example,perpendicular to the glass sheet. Preferably, δ is in the range fromabout 15 degrees to about 75 degrees, preferably in the range from about25 degrees to about 65 degrees, and more preferably in the range fromabout 35 degrees to about 55 degrees. In one embodiment, the angle ofthe air flow is about 45 degrees relative to the vertical X-Y plane. Itshould be noted that the preferred direction for the air flow isdownward, since a low positioning of the air bearing relative to theglass sheet gives the lower portion of the glass sheet more stiffness toresist buckling of the glass sheet due to the air flow. However, in someembodiments, an upward air flow may be preferred depending on processconditions and the particular implementation, e.g. positioning, of theair bearing.

The preceding description presented three optional orientations of anexemplary stabilizing air knife 120. Each stabilizing air knife of theplurality of stabilizing air knives may exhibit at least one orientationof the three optional orientations described above in respect of arepresentative exemplary stabilizing air knife. For example, eachstabilizing air knife of the plurality of stabilizing air knives mayexhaust air such that the direction of air flow from the air knives isgenerally downward (i.e. the flow vector comprises a vertical vectorcomponent). Thus, for example, two stabilizing air knives located onopposite sides of the glass sheet and wherein the stabilizing air knivesare mirror images of each other, will form a generally V-shaped flow ofair, with the “V” pointed downward.

Similarly, each stabilizing air knife of the plurality of stabilizingair knives may be oriented such that a leading end of each stabilizingair knife is farther from the glass sheet than a trailing end. Thus, forexample, two stabilizing air knives located on opposite sides of theglass sheet and wherein the stabilizing air knives are mirror images ofeach other, will form a generally V-shaped flow of air, with the “V”pointed downstream toward the air bearing. This provides more lateralclearance for a glass sheet exhibiting lateral movement as it entersbetween the air knives. It also provides for a more gradual applicationof the curtain of air flowing from the stabilizing air knives, as thepressure on the glass sheet from the flow of air from the leading end ofeach stabilizing air knife becomes less than the pressure of the air onthe glass sheet adjacent the trailing end of a stabilizing air knife.

Similarly, each stabilizing air knife of the plurality of stabilizingair knives may be oriented such that a leading end of each stabilizingair knife is lower relative to a horizontal reference plane (forexample, the X-Z plane) than a trailing end. It can be said that thestabilizing air knives are pitched or inclined forward to flatten outany shape distortion (e.g. bow) in the sheet.

Each stabilizing air knife of the plurality of air knives may exhibitone or more of the orientations described above. In some embodiments,one or more of the stabilizing air knives may simultaneously exhibit allthree orientations.

In addition to the stabilizing air knives and as seen in FIG. 3, forexample, a first positioning air knife 132 may be placed between thestabilizing air knives (52 a, 52 b) and air bearing 44 such that theflow 126 of air from the first positioning air knife impinges on firstmajor surface 121 of the glass sheet adjacent to the leading edge of theair bearing. For example, first positioning air knife 132 may be locatedat an approximately 270 degree position on air bearing 44. The pressureproduced on the glass sheet as it passes adjacent to first positioningair knife 132 forces the glass sheet away from the air bearing. Thisprevents contact between the leading or forward edge of the glass sheetas it approaches the air bearing until the glass sheet can be “captured”by the air bearing.

A second positioning air knife 134 may be positioned such that air fromthe second positioning air knife impinges on second major surface 123 ofthe glass sheet. The effect of the air from the second positioning airknife is to force the glass sheet in a direction toward the air bearing,thus bringing the glass sheet closer to the air bearing and allowing theair bearing to capture the glass sheet. Initial capture of the glasssheet is accomplished by the combination of pressure and vacuum producedby the outer porous body portion.

A third positioning air knife 136 may be positioned downstream from theair bearing and positioned such that air emitted by the thirdpositioning air knife is directed against first major surface 121 of theglass sheet. The air pressure produced by third positioning air knife136 forces the glass sheet away from the air bearing surface near thedownstream edge of the air bearing and thereby prevents contact betweenthe glass sheet and the air bearing as the glass sheet moves past anddisengages from the air bearing. Each positioning air knife 132, 134 and136 may be similar in design to a stabilizing air knife. For example,each stabilizing air knife and each positioning air knife may be of thearcuate design or of the linear design. Preferably, the air emitted fromeach of the positioning air knives 132, 134 and 136 is directed againstthe glass sheet such that the curtain of gas from each positioning airknife forms an angle less than 90 degrees but greater than zero with thesurface of the glass sheet, for example, greater than 25 degrees andless than 75 degrees, and preferably greater than 35 degrees and lessthan 65 degrees, preferably greater than 35 degrees and less than 55degrees. For example, a typical embodiment may orient each positioningair knife so that the flow of air impinges on the glass sheet at anangle of about 45 degrees. An angle of impingement less than 90 degreesproduces less turbulence at the surface of the glass sheet than, forexample, air flow that is perpendicular to the glass sheet.

The overall effect of the various non-contact glass sheet handlingcomponents of apparatus 40 is to provide gradually increasing constrainton the glass sheet to prepare the glass sheet for measurement. Aspreviously noted, in some instances the glass sheet is conveyedvertically, secured only at the top of the glass sheet by the conveyorclamp. As the glass sheet may be very thin, equal to or less than 1 mm,and in some cases equal to or less than 0.7 mm, or in other cases equalto or less than 0.3 mm, the glass may easily exhibit lateral movement byswaying side-to-side (i.e. rotate about the fixed carrier contactpoints), or deform by various bending modes (as used herein, a bendingmode is analogous to a vibrational mode). The glass can also be offsetdue to carrier-to-carrier variations in the clamping and the position ofthe carrier on the conveyor. As well, the glass can be bowed vertically.

Various glass sheet handling components of apparatus 40 serve to reduceor eliminate these motions, and fixed shapes, such as bow. Accordingly,operation of apparatus 40 may proceed along the following steps.

A glass sheet 28 is attached to conveyor 48 by one or more clampingmechanisms 49 that grip the glass sheet along a top edge of the glasssheet translate the glass sheet through apparatus 40. Glass sheet 28 isthereby hanging from the one or more clamping mechanisms, and supportedonly by the one or more clamping mechanisms clamped to the glass sheetalong a top portion of the glass sheet. The lower edge 63 of the glasssheet is unsupported and initially capable of lateral movement, i.e. aswaying movement, before entering apparatus 40. In addition to lateralmovement, the glass sheet may also exhibit flexure or bending. Forexample, the sheet may bend cylindrically or hyperbolically or be saddleshaped, dome shaped, or exhibit other bending modes, or combinationsthereof.

As glass sheet 28 nears apparatus 40, the glass sheet is guided by atleast one edge guiding device 54 that engages with lower edge 63 of theglass sheet and guides the leading edge of the glass sheet betweenstabilizing air knives 52 a, 52 b. Lower edge 63 forms part of the“non-quality” portion of the glass sheet and may later be removed. Theat least one edge guiding device 54 minimizes or eliminates side-to-sideswaying. Testing has shown embodiments of edge guiding device 54 asdisclosed herein can reduce the lateral motion of swaying from a maximumdisplacement of +/−75 mm to less than +/−10 mm. However, while the atleast one edge guiding device 54 may provide excellent control oflateral movement of the lower edge of the glass sheet, the glass sheetis only substantially constrained at the top and bottom edges, and isstill capable of exhibiting various bending modes and fixed shapeswithin the body of the glass sheet. To minimize or eliminate thisadditional movement or shape of the glass sheet before moving adjacentto porous body 84, stabilizing air knives are employed.

The flow of air emitted by opposing stabilizing air knives, preferablyin a downward direction, may further reduce lateral movement of theglass sheet to eliminate side-to-side swaying of the glass sheet, and inparticular, reduces or eliminates bending modes. In effect, thestabilizing air knives help stiffen the glass sheet by at least reducingthe magnitude of the bending and in some cases by eliminating one ormore bending modes. The number and positioning of the stabilizing airknives is dependent on such factors as the size of the glass sheet, thethickness of the glass sheet, the density of the glass, and the traversespeed of the glass sheet through apparatus 40.

As the glass sheet passes between the stabilizing air knives, lower edge63 of the glass sheet may be guided by one or more additional edgeguiding devices 54 to further guide and stabilize the glass sheet. Forexample, in some embodiments, multiple edge guiding devices may beemployed; with a first edge guiding device employed upstream of thestabilizing air knives and a second edge guiding device positioned justprior to the edge constraining device 62.

As the glass sheet approaches air bearing 44, optional first positioningair knife 132 may be used to direct a flow of air at first major surface121 of the glass sheet. The force of the air from first positioning airknife 132 against first major surface 121 of glass sheet 28 pushes theglass sheet away from the leading edge 140 (see FIG. 17, and moreparticularly region A of FIG. 18) of air bearing 44 and prevents contactbetween leading edge 140 of the air bearing and leading edge 141 ofglass sheet 28 as the glass sheet comes under the influence of outerporous body portion 92 of air bearing 44. Contact between the glasssheet and the air bearing may result in damage, in some instancescatastrophic, to the glass sheet.

As the glass sheet continues to move forward along direction of travel50, the glass sheet passes over a first vacuum port 118 of outer porousbody portion 92. Preferably, air bearing 44 is positioned such that avacuum port 118 positioned within the outer-most groove of outer porousbody portion 92 is positioned so that as the glass sheet advances, itfirst moves adjacent to this single vacuum port 118. Referring to FIG.10, this first vacuum port 118 is the vacuum port farthest to the leftand lying in the outermost continuous groove 116 in FIG. 8, andintersecting with dashed line 119. The effect of this initial encounterwith a first vacuum port 118 is that leading edge 141 of glass sheet 28is moved closer to outer porous body portion 92. That is, while theregion of the glass sheet adjacent leading edge 141 of the air bearingis being pushed away from the air bearing leading edge, a region of theglass sheet adjacent the first vacuum port 118 is forced in thedirection of outer porous body portion 92. By bringing at least thisportion of the glass sheet adjacent the first vacuum port 118 close tothe air bearing, that portion of the glass sheet is captured by theouter porous body portion 92 of air bearing 44. Continued forwardmovement of the glass sheet brings the glass sheet adjacent toadditional vacuum ports 118 of outer porous body portion 92. Within ashort distance of the leading edge of glass sheet 28 passing adjacent tothe additional outer porous body portion vacuum ports 118, sufficientforce is exerted on the glass sheet as a result of air flow at the outerporous body portion that a substantial portion of the glass sheetadjacent to air bearing 44 exhibits a substantially uniform fly heightrelative to the first major surface of the air bearing.

Again, as the glass sheet continues to move forward adjacent to innerporous body portion 90, the flatness and rigidity of the glass sheetincreases, particularly those portions of glass sheet 28 directlyadjacent to inner porous body portion 90, such that measurements may betaken by measurement device 104 through passage 102.

It should be recalled that measurements of the glass sheet, such asinterferometric measurements for the purpose of determining surfacetopography of glass sheet 28, may be taken simultaneously with theforward movement of the glass sheet (i.e. in direction 50). As the glasssheet trailing edge passes inner porous body portion 90, and then outerporous body portion 92, the constraint to the glass sheet applied by theaction of the air bearing 44 decreases, and the air pressure fromoptional positioning air knife 136 is able to overcome the holding forceapplied by air bearing 44 such that the trailing edge of the glass sheetis pushed away from the air bearing so that contact between the glasssheet and the air bearing does not occur. The pressurized air suppliedto inner porous body portion 90, and the vacuum, may be adjusted, forexample, such that the fly height of the glass sheet is maintained tohave a deviation less than about 30 μm (+/−15 μm).

From the preceding it can be seen that a region of the glass sheet isopposite passage 102 as the glass sheet moves past air bearing 44. Thus,passage 102, while defining a circular measurement zone, “sweeps” arectangular measurement region 138 of the glass sheet as shown in FIG.9. Measurement device 104 makes continuous measurements of the glasswithin this rectangular region. For example, measurement device 104 maybe an interferometer for making surface topography measurements of theglass sheet within the rectangular measurement zone, or measurementdevice 104 may make measurement of the thickness of the glass sheet.

Eventually, continued forward travel of the glass sheet along directionof travel 50 brings the trailing edge 142 of the glass sheet past airbearing 44. Air issuing from third positioning air knife 136 andimpinging on first major surface 121 of the glass sheet forces theregion of the glass sheet proximate the trailing edge 142 of air bearing44 away from the air bearing, thereby avoiding contact between the glasssheet and the air bearing surface. This becomes particularly beneficialas the surface area of the glass sheet under the influence of thestabilizing air knives and/or the air bearing decreases.

The result of the forces supplied by positioning air knives 132, 134 and136 can be seen with the aid of FIG. 18, showing apparatus 40 in a topview and depicting glass sheet 28. The effect of positioning air knives132 and 136 can be seen in the regions designated by reference numeralsA and B and circled by dashed lines. Indeed, it can be seen from FIG. 18that the glass sheet overall takes on a non-planar aspect until theglass sheet is fully engaged by the air bearing, at which point theglass sheet adjacent to the air bearing is fully flat, although itshould be noted that the only region where planarity is needed is wherea measurement is occurring, e.g. in the center of the measurement zone.

FIG. 19 depicts an edge view of the glass sheet extending from thecenter of the inner porous body portion to the leading edge of the airbearing and shows in more detail the shape of the glass sheet over theair bearing. It should be kept in mind that the illustration of FIG. 19is greatly exaggerated, as the deflections involved are on the order oftens of microns. As shown, the glass sheet can be divided into severaldistinct regions separated by equally distinct boundaries, giving theportion of the glass sheet over the air bearing the appearance of aseries of relatively flat plateaus 144 separated by S-shaped boundaryfeatures 145, wherein the fly height of the plateaus decrease in adirection toward the center of the inner porous body portion (e.g. thearea defined by passage 102).

FIG. 20 is a graph of measured fly heights for a glass sheet travelingthrough an embodiment of apparatus 40 with a travel speed of 100 mm/smeasured at two different locations for a glass sheet. The vertical “Y”axis represents fly height in microns while the horizontal “X” axisrepresents time. The measurements were taken at a predeterminedfrequency (250 sec⁻¹) for a given location. Thus, the graph of FIG. 20can be used to obtain the fly height at a predetermined position overtime as the glass sheet traverses adjacent to the air bearing. The glasssheet had an initial side-to-side sway of +/−75 mm from a nominalcenterline position. Each stabilizing air knife directed air against theglass sheet at a downward angle of 45 degrees. The inner porous bodyportion of the air bearing was supplied with air at a pressure of 20 psito 60 psi at a flow rate of 0.63+/−0.25 CFM, while the outer porous bodyportion was supplied with air at a pressure of 40 to 85 psi and a flowrate of 0.96+/−0.35 CFM. The fly height of the glass sheet over thecircular measurement zone defined by passage 102 was nominally 28 μmwith a variation of less than +/−2.5 μm. Curve 146 of FIG. 20 shows thefly height of the glass sheet at the outer circumference of inner porousbody member 90 at a position of approximately 210 degrees, with theglass sheet traveling at a speed of 100 mm/s, while curve 148 of FIG. 20depicts the fly height at the center of the inner porous body portion(i.e. the center of passage 102). The curves for travel of the glasssheet are particularly telling for showing the stability of the flyheight at both locations of the glass sheet—the measurement location atthe outer circumference of the outer porous body portion and themeasurement location over the center of the porous body. While thenominal fly height differs between the two regions, being significantlygreater for the outer porous body location than for the center of theporous body, the fly height at both locations is surprisingly stable,showing a variation less than about ±2.5 microns.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of characterizing moving glass sheetscomprising: moving a glass sheet in a first direction along apredetermined path past a circular air bearing, the glass sheetcomprising a pair of opposing major surfaces and a bottom edge;dampening movement of the glass sheet in a second directionperpendicular to the first direction by passing the glass sheet betweenat least two stabilizing air knives positioned adjacent the opposingmajor surfaces of the glass sheet as the glass sheet is moving past thecircular air bearing; engaging the glass sheet with the circular airbearing, the circular air bearing comprising an inner porous bodyportion and an outer porous body portion disposed about the inner porousbody portion, the inner porous body portion and the outer porous bodyportion positioned adjacent at least a portion of one of the pair ofopposing major surfaces as the glass sheet is moving past the circularair bearing, the inner porous body portion defining a central passagetherethrough; and measuring at least one attribute of the glass sheetthrough the central passage as the glass sheet is moving past thecircular air bearing.
 2. The method according to claim 1, furthercomprising flattening at least a portion of the glass sheet.
 3. Themethod according to claim 1, wherein the glass sheet is verticallysuspended during the moving.
 4. The method according to claim 1, furthercomprising guiding the bottom edge of the glass sheet with an edgeguiding device comprising guide arms arranged to form a “V”-shaped slottherebetween.
 5. The method according to claim 1, wherein a height ofthe circular air bearing is less than one half a height of the glasssheet.
 6. The method according to claim 1, wherein the circular airbearing is positioned such that an upper one half of the glass sheet isnot adjacent to the circular air bearing as the glass sheet is measured.7. The method according to claim 1, wherein the circular air bearingmaintains the glass sheet within +/−15 μm of a predetermined distancefrom the inner porous body portion.
 8. The method according claim 1,wherein the inner porous body portion comprises a circular groove in asurface thereof, and a plurality of radial grooves intersecting thecircular groove.
 9. The method according to claim 1, wherein the outerporous body portion of the circular air bearing comprises a plurality ofconcentric grooves, each groove of the plurality of concentric groovescomprising a plurality of vacuum ports, the method further comprisingapplying a vacuum to the plurality of vacuum ports.
 10. A method ofcharacterizing moving glass sheets comprising: moving a glass sheet in afirst direction past an arcuate air bearing; dampening movement of theglass sheet in a second direction perpendicular to the first directionby passing the glass sheet between at least two stabilizing air knivespositioned upstream of the arcuate air bearing relative to the firstdirection as the glass sheet is moving in the first direction; engagingthe glass sheet with the arcuate air bearing, the arcuate air bearingcomprising an inner porous body portion and an outer porous body portiondisposed about the inner porous body portion, the arcuate air bearingproducing a low precision capture zone between the outer porous bodyportion and the glass sheet, and a high precision capture zone betweenthe inner porous body portion and the glass sheet such that a fly heightof the glass sheet in the low precision capture zone is greater than afly height of the glass sheet in the high precision capture zone as theglass sheet moves past the arcuate air bearing; and measuring at leastone attribute of the glass sheet through a passage extending through theinner porous body portion as the glass sheet is moving past the arcuateair bearing.
 11. The method of claim 10, wherein the inner porous bodyportion and the outer porous body portion are supplied with apressurized gas that issues from surfaces of the inner porous bodyportion and the outer porous body portion.
 12. The method according toclaim 11, wherein a vacuum is applied to vacuum ports positioned withingrooves located in the surfaces of the inner porous body portion and theouter porous body portion.
 13. The method of claim 10, wherein thearcuate air bearing comprises an oval, a circular or an ellipticalshape.
 14. The method of claim 10, wherein the at least two stabilizingair knives direct a flow of gas in a downward direction.
 15. The methodaccording to claim 10, further comprising forcing the glass sheet awayfrom the arcuate air bearing with a positioning air knife locateddownstream of the arcuate air bearing relative to the first direction.16. The method according to claim 10, wherein the measured attribute isat least one of cord, streak or a thickness of the glass sheet.
 17. Themethod according to claim 10, wherein the moving comprises clamping theglass sheet at an upper edge of the glass sheet with a clampingmechanism such that the glass sheet hangs from the clamping mechanism.18. The method according to claim 10, further comprising capturing abottom edge of the glass sheet in a guide slot.
 19. The method accordingto claim 18, further comprising constraining movement of the bottom edgein the second direction with a plurality of guide rollers positionedadjacent the opposing surfaces of the glass sheet and downstream fromthe guide slot as the glass sheet moves past the arcuate air bearing.20. The method according to claim 10, wherein the fly height of theglass sheet adjacent the high precision capture zone is less than 40micrometers.